Numerous methods and apparatuses have been developed for the purpose of measuring flow of a fluid. The accurate measurement of the flow of a fluid presents many varied design problems depending upon the type of fluid in the particular application. In some instances, flow can be measured directly by checking the weight or volume of the fluid flow for a given interval of time, or by utilizing a positive displacement meter. Flow may also be measured indirectly using obstruction meters, velocity probes, or a variety of highly specialized meters such as anemometers, coriolis effect flow meters, sonic flow meters and magnetic flow meters. Choosing a flow meter is always a compromise, and a number of factors have to be carefully considered, namely, cost, size, range, types of fluid, form of output, and accuracy.
There are many applications, such as commercial and residential gas meters, where it is important to accurately measure total flow over a specific time interval during which flow may vary widely from the extremely small flow rate required by a pilot light to a maximum flow rate which may occur when all of the gas powered appliances are operating simultaneously. When using positive displacement pumps as is currently the norm in gas measurement, high accuracy can be achieved at both the high and extremely low flow rates. With the current trend toward supplying remotely readable gas meters, it is necessary to add electrical output devices to positive displacement flow meters, thereby greatly increasing their cost. The principal draw-back preventing use of indirect flow measurement devices has been the lack of accuracy throughout the wide operating range of flows. Most indirect flow meters capable of accurately measuring pilot light flow are too restrictive at high flow rates, and conversely flow meters capable of accurately measuring high flow rates cannot accurately sense extremely low flows.